Abrasive materials may be either natural or synthetic. Traditional abrasives are all natural, and the synthetic ones are a fairly recent innovation.

The oldest abrasive of all is sand, which was used for polishing stone weapons as early as 25,000 BC. Other abrasive materials in use from early times include garnet (a hard, glasslike gemstone), emery, pumice, and silica (silicon dioxide) which occurs in various forms as quartz, flint and agate. In the Middle Ages, grinding wheels of quartz and flint fragments naturally bonded together in rock were used. Gemstones were lapped or polished by the use of emery or sandstone powder rubbed on with metal plates.

The abrading effect is produced almost entirely by the simple physical process of the harder substance shearing or fracturing small chips off the work-piece to smooth it.

Abrasives are used in three main ways. One is to use the abrasive material directly on a substance: sharpening a knife on a grinding wheel is an example of this. Another is to coat another substance, such as a piece of paper, cloth or rubber on a metal disc, with granules of abrasive material, and use this as a tool; sandpaper is the commonest application of this technique. The third method is sandblasting or gritblasting, where a powerful stream of air containing abrasive particles is directed at an object to abrade its surface.

The flash of the Hiroshima bomb was so intense that it discolored concrete and sealed the surface of granite, leaving in many places prints of the shadows cast by the light of the explosion. By triangulating these shadows with the objects that had cast them, Japanese scientists were able to pinpoint the exact center of the blast. Some of the shadows were of people.

Today’s nuclear warheads are smaller and more powerful than ever before, in order to maximize the efficiency of the delivery system. At the outset of the Manhattan Project, Albert Einstein was one of the scientists who forecast that an A-Bomb would have to be so large and heavy that it would need a ship to deliver it to its target.

Uranium is not the only material used for making A-bombs. Another material is the element plutonium, in its isotope Pu-239. Plutonium is not found naturally (except in minute traces) and is always made from uranium. This can be done by putting U-238 in a nuclear reactor. After a while, the intense radioactivity causes it to pick up the extra particles, so that more and more of its atoms turn into plutonium.

Plutonium will not start a fast chain reaction by itself, but this difficulty is overcome by having a neutron source, a highly radioactive material that gives off neutrons faster than the plutonium itself.

The A-bomb, or atomic bomb, has been used only twice in war, at Hiroshima and Nagasaki, and both times it has totally destroyed a large city in a single explosion.

Strictly speaking, the term atomic bomb includes the H-bomb, which also uses atomic power. But in general usage, atomic bomb is reserved for earlier weapons that work by nuclear fission, that is, splitting atoms. The hydrogen bomb works by nuclear fusion - joining small atoms together to make larger ones. Both fission and fusion release huge amounts of energy and radioactivity.

Nuclear fission